Along the Great Divide (The Travelers) (1951) [Ed Roden]: Shot repeatedly by James Anderson. (Thanks to Gordon)

The Man Behind the Gun(1953) [Bram Creegan]: Killed (off-screen) by Roy Roberts' men; we learn of his death afterwards when Randolph Scott digs up the grave supposedly belonging to Roy (who had faked his death) and reports finding Morris' body.

Cattle Queen of Montana(1954) [J.I. 'Pop' Jones]: Trampled to death in a stampede caused by Native American warriors. (Thanks to Gordon)

Earth vs. the Flying Saucers (Invasion of the Flying Saucers) (1956) [Brigadier General John Hanley]: Thrown out of the spaceship by the aliens, after they telepathically drain his mind of information. (Thanks to Stephen)

Drango(1957) [Henry Calder]: Hanged by a lynch mob led by Ronald Howard. (Thanks to Gordon)